The application of a programmed digital computer control system and method for the control of slab heating, particularly for operation in conjunction with an associated hot strip mill, is desirable because of the potential economic saving of considerable dollars. The heating of slabs involves the use of very large amounts of heat energy and the related production of very large total tonnages of steel when integrated over a calendar period such as one year. Improvements in overall consumption of fuel and in scale loss of the metal can be realized and evaluated into dollar figures which far outweigh any investment required to appy such a computer control system. In addition, improvements in slab heating furnace operation and in better overall control of product quality can be realized. The cost advantages and savings in dollars are multiplied to a greater extent as the number of furnaces provided to match the rolling capacity of the associated rolling mills is higher in number and the resultant control actions necessaary to obtain properly heated slabs are sufficient in magnitude to go beyond the ability of a human furnace operator to handle the wide variety of situations which can arise. The availability of high speed programmed digital computer control techniques permits very fast analysis of monitored changes in operating conditions and the desired resultant corrective furnace control operations to follow changes in rolling mill operation faster and more efficiently.
This invention pertains to the operation of a slab heating furnace and, more particularly, to controlling the operation of a slab reheat furnace used to heat metal slabs in preparation for rolling in a hot strip rolling mill.
To satisfy the demands of hot strip mills, it is necessary to heat slabs to a desired temperature in continuous furnaces, which heat is supplied to each slab on both its upper and lower surfaces as it passes through predetermined heating zones of the furnace. Generally, the slabs are transported in a direction opposite to that of the flow of the heated gases, with the furnaces being divided into predetermined heating zones wherein each heating zone has its own temperature controller for the firing rate of the one or more burners in each of the heating zones. For example, in the conventional five heat zone pusher furnace, three heating zones are utilized and conventionally these are called preheat or charge, heat and soak.
The preheat or charge zone is most generally the largest of the three heating zones, both in terms of physical size and in amount of heat added to the slab; it has separate burner systems for the upper and lower slab surfaces. The heat zone which follows the charge zone than heats the slab to the desired rolling temperatures using a similar burner system. When the slab surfaces have reached a desired temperature, the slabs are then transported into the soak zone for the purpose of providing a uniform temperature profile distribution throughout the slab.
Because of the continuous nature of material passage through these furnaces, it is highly desirable to approach an operating condition wherein the steady state heated slab output of the furnace substantially matches the heated slab input requirements of the associated rolling mill. Also, it would be advantageous to provide optimum operating conditions in the slab heating furnace to provide the most economical and satisfactory operation of the whole associated process. In addition to economic advantages, improvements in furnace operation and better overall control of heated slab quality will be realized as compared to the prior art conventional and existing modes of furnace operation.
It is desired to provide a uniformly heated slab at the proper value of rolling temperature for a wide variety of slab sizes, mixes of grades of slabs, and rates of individual slab movement. Wide fluctuations and sudden changes in firing rates for the respective zones should be minimized in meeting the demands of the associated rolling mill. The soak time requirements in the soak zone for each slab should be set at a minimum consistent with maximum production and minimizing of skid marks, by assuring that the slab has the required total heat content before entering the soak zone area of each furnace. Minimum furnace temperatures should be provided corresponding to the slab tonnage rates being demanded by the rolling mill and the average slab thickness proceeding through each zone of each furnace. It is desired to distribute firing rates between individual firing zones for most efficient utilization of provided heat energy, and multizone continuous furnaces are designed with this capability in relation to variation in demand of the associated rolling mill. The slabs should be protected from damage due to excessive heating, scaling, and so forth when demands by the rolling mill for heated slabs change rapidly. Increased life and lower maintenance on individual furnace refractories can be realized by minizing effects of changes in demand for heated slabs. A control system and method should be provided that is flexible and expandable to permit furnace response to zone temperatures measured and operational feed back from key locations in the associated rolling mill; for example, slab temperatures measured in relation to the initial pass through an early stand of the hot strip rolling mill. The control approach should be applicable to any multizone controlled continuous type slab heating apparatus, such as a pusher, walking beam or roller hearth types of furnaces, or to combinations of these types, and also be applicable to combination fuel fired and electric furnace systems.
A previously filed patent applicaion, Ser. No. 705,506 and filing date Feb. 14, 1968, was filed by the same inventors to conver an earlier control system for slab reheat furnaces, which included sensing the physical movement of the slabs as well as monitoring of mill pace time and temperature sensing devices for providing temperature feedback signals related to the upper and lower surface temperatures of the slab. This application has been abandoned.
A digital process control computer can include a central integrated process control or setup processor operative with a software sequentially stepped instruction program which is entered into and stored within the storage memory unit of the computer, and including associated input and output equipment such as generally described in an article entitled "Understanding Digital Computer Process Control" by B. H. Murphy, which appeared in Automation for January 1965, pages 71 to 76, and in an article entitled "Small Control Computers--A New Concept" by F. G. Willard which appeared in the Westinghouse Engineer for November 1964 at pages 174 to 179. Two other articles of interest here in regard to the programming of a process control computer should also be noted; one was published in the January 1965 Westinghouse Engineer at pages 13 to 19 by Paul E. Lego and the other was published in the 1966 Iron and Steel Engineer Year Book at pages 328 to 334 by J. S. Deliyannides and A. H. Green. Each computer processor is associated with predetermined input systems not specifically shown, which typically include an input system which scans process signals representing the status of various process operating conditions, a conventional analog input system which scans and converts process analog signals and operator controlled and other input devices and systems which could include paper tape, teletypewriter and dial input apparatus. Various kinds of information are entered into the computer control system through information input devices including for the example of a slab heating furnace, the desired slab heat content H.sub.T, grade of slab being heated and so forth as well as hardware oriented programs and control programs for the programming system and so forth. The input system interfaces the computer control system with the process through the medium of measured or detected signals. To effect desired output control actions, control devices are operated directly by means of an output system or by means of analog signals derived from the output system through a digital to analog converter. One such control action outputs from the computer control system the temperature setpoint for each heating zone which are applied to the respective temperature controllers to determine the desired operation of the heating zones. The previously determined slab heat content values for each heating zone are stored in memory along with the calculated values of other selected operational parameters. A suitable output display can be provided for operation with the computer control system in order to keep the process operator generally informed about the process operation and in order to signal the operator regarding an event or condition relative to any particular furnace which may require some action on his part. The use of an on-line digital computer control system requires that one or more models relating to the controlled process be stored in the memory unit of the computer to enable predictive operation and control of the process and adaptive control of the process relative to updating information obtained from actual operation of the process.